Dear DB,
The figures in your correspondence suggest there are two separate questions: how your UK state pension has been calculated, and whether your years working in Belgium entitle you to a separate Belgian pension.
Starting with your UK state pension, the original estimate of £260.66 a week provided to you by the Pension Service appears to consist of the full new state pension of £241.30 plus a protected payment of £19.36.
A protected payment is an additional amount some people receive because they had built up more pension under the old state pension system before the new state pension was introduced in April 2016. Rather than losing that entitlement, it is paid on top of the full new state pension.
That’s why your later correspondence is confusing. It states your pension will be £241.30 a week while also saying this figure includes a protected payment of £19.36. To me, those numbers don’t add up. If you are entitled to a protected payment of £19.36, your total weekly state pension should be £260.66.
This may simply be an administrative or wording error, but you should ask the Pension Service for a written breakdown showing your standard state pension, any protected payment and your total weekly entitlement. If it cannot reconcile the figures in writing, ask for a formal state pension calculation. That should clearly show how your entitlement has been worked out and whether a protected payment has been included.